A known joint structure in the related art includes a double seal member that can seal a gap between relatively moving surfaces by lip contact to prevent the entrance of dust and water drops into mechanical components, such as a reduction gear and a bearing, provided inside a joint and to prevent leakage of lubricant from the mechanical components (see, for example, PTL 1).
With this joint structure, for example, for the purpose of cleaning a workpiece that is formed of machine processed components by exposing the workpiece to a fixed washing-liquid spraying nozzle, even when the workpiece is strongly hit with droplets of washing liquid sprayed from the washing-liquid spraying nozzle, the sealed state established by an inner seal member can be maintained unbroken.